This invention relates to methods for delivering beneficial microorganisms to seeds and plants and to seeds and plants containing such microorganisms. More particularly, the invention relates to seeds and plants containing viable beneficial microorganisms, methods for introducing such microorganisms into seeds and plants, and methods for colonizing plants with beneficial microorganisms by introducing the microorganisms into the seeds and plants.
It is a well-known fact that many types of microorganisms are harmful to seeds and plants. However, many types of microorganisms are also beneficial to plants. Nitrogen-fixing bacteria, such as Rhizobium spp., colonize the roots of legumes and provide nitrogen in a form usable by those plants. Certain other microorganisms, known as biological control agents, appear to provide some seeds and plants with a degree of protection against harmful microorganisms, when they are associated with those seeds and plants. For example, it has been shown that Trichoderma spp. can sometimes protect seeds as effectively as chemical fungicides when coated onto the seeds. Chao, et al., Phytopathology, 76:60-65 (1986). Still other microorganisms, such as the hybrid agricultural-chemical-producing endosymbiotic microorganisms disclosed in U.S. patent application Ser. No. 08/178617, which is incorporated herein by reference in its entirety, colonize the interior of plants in a symbiotic manner and provide useful agricultural chemicals, such as pesticides, to them.
Colonization of plants and seeds with beneficial microorganisms has conventionally been accomplished by applying the microorganisms directly to the surface of the seeds or to the soil surrounding the plants. With Rhizobium, for example, the microorganisms are usually coated onto the seeds, and they colonize the roots and rhizosphere of the plants as the plants emerge from the seeds. See U.S. Pat. No. 4,434,231 to Jung, U.S. Pat. No. 4,367,609 to Lloyd, U.S. Pat. No. 3,054,219 to Porter et al., and U.S. Pat. No. 2,995,867 to Burton. Similarly, the biocontrol agent Trichoderma is applied by coating it onto seeds. See Chao et al., supra, and Hadar et al., Phytopathology, 74:106-110 (1984). Alternatively, beneficial microorganisms have been applied to plants by preparing a water-based suspension of the microorganisms and spraying that suspension onto the plants or applying it directly to the soil around the plants. All of these methods have been used to colonize the surface, as opposed to the interior, of plants with beneficial microorganisms.
Seed coating and soil application techniques have at least two disadvantages. First, certain microorganisms will not survive well in the soil and will not colonize the roots or other parts of the plant as it emerges from the seed, unless the plant is mechanically or otherwise wounded to provide an entry to the microorganisms. Therefore, merely coating these microorganisms onto seeds will not cause colonization of the resultant plant. Second, even for microorganisms which can colonize plants by virtue of being applied to the surface of the seeds, there can be a natural loss of viability of some of the microorganisms and the efficiency of the colonization can be low.
The present inventor undertook to develop a method of colonizing plants and seeds with beneficial microorganisms that would overcome these disadvantages. It was known that pathogenic microorganisms could be impregnated into seeds by preparing an aqueous suspension of the pathogens, mixing the seeds with the pathogens, and using the technique of vacuum infiltration to force the pathogens into the seeds. See Leben, Plant Disease, 65:876-878 (1981) and Goth, Plant Disease Reporter, 50:110-111 (1966). Pathogenic microorganisms have also been inoculated into plants by vacuum infiltration (Rowell and DeVay, Phytopathology, 43:654-658 (1953)) and by means of a needleless medical jet injector (Wastie, Plant Pathology, 33:61-63 (1984)). However, such techniques lead to the destruction of the seed or the resultant plant by the pathogen.
The present inventor has discovered that beneficial microorganisms can be impregnated into seeds, whereby the microorganisms and seeds remain viable and the resulting plants are viable and colonized by the microorganisms. This is surprising in view of the fact that certain microorganisms, which are not themselves pathogenic to a particular plant species, can attack and destroy seeds of that species either pre- or post-harvest, while not causing disease in the plant itself. The present inventor has also discovered that certain beneficial microorganisms, which might have been expected to colonize plants by merely being applied to the surface of the plants, need to be physically injected into or otherwise caused to enter the plants for colonization to occur. The inventor has developed various techniques to implement these discoveries.